In the name of Allah, Most Compassionate, Most Merciful,
Generally, it is impermissible for a woman to travel the distance of three days (equivalent to 48 miles) without her husband or a Mahram (unmarriageable kin) accompanying her.
There are many clear narrations of the Messenger of Allah (Allah bless him & give him peace) in this regard.
1) Sayyiduna Abu Sa’id al-Khudri (Allah be pleased with him) narrates that the Messenger of Allah (Allah bless him & give him peace) said: “Let no woman travel for more than three days unless her husband or a Mahram is with her.” (Sahih Muslim)
2) Sayyiduna Abd Allah ibn Umar (Allah be pleased with him) narrates that the Messenger of Allah (Allah bless him & give him peace) said: “A woman must not travel for three days except with a Mahram.” (Sahih al-Bukhari, no. 1036 & Sahih Muslim)
3) Sayyiduna Abu Huraira (Allah be pleased with him) narrates that the Messenger of Allah (Allah bless him & give him peace) said: “It is unlawful for a woman who believes in Allah and the last day that she travels the distance of one day and one night without a Mahram accompanying her.” (Sahih al-Bukhari, no. 1038)
4) Sayyiduna Ibn Abbas (Allah be pleased with him) narrates that the Messenger of Allah (Allah bless him & give him peace) said: “’A woman must not travel except with a Mahram and a man must not enter upon her except if she has a Mahram.” (Sahih al-Bukhari, no. 1763)
Imam Nawawi (may Allah have mercy on him) states in his monumental commentary of Sahih Muslim:
“There are many such narrations that assert the impermissibility of a woman travelling without a Mahram. These narrations vary in their wordings. The narration of Ibn Abbas in Sahih al-Bukhari says that a woman must not travel without a Mahram, but it adds nothing else. However, the other narrations, in Sahih al-Bukhari and elsewhere, mention lengths of journeys for which a Mahram is required – some of the narrations specify three days, some two, some one, and some even less.”
Imam Nawawi (Allah have mercy on him) also mentions that the difference found in these narrations is due to the different questioners and the places wherein the answers were given to them. al-Bayhaqi said: “It is as though the messenger of Allah (Allah bless him & give him peace) was asked regarding travelling for three days without a Mahram, and he refused. He was then asked about her travelling for two days, and regarding one day, etc and each narrator related from him what he heard.” (See: Commentary of Sahih Muslim by Imam Nawawi, 1015)
According to the Hanafi Ijtihad, the distance that is considered here is three days and three nights, for the narrations mentioning three days & three nights have reached the level of certainty. All the Companions who narrated other than three days also narrate the distance of three days and three nights. The narrations that mention two or one day will be restricted to specific circumstances, such as the fear of more fitna. Hence, they (Hanafi School) considers the narrations that mention three days & three nights as the basis of prohibition. (Zafar Ahmad al-Tahanawi, I’la al-Sunan, V. 10, P. 11)
It must be remarked here that this refers to the distance usually covered by walking or on an animal in three days & three nights (with the usual breaks for resting and eating). Therefore, the restriction of travelling with a Mahram applies if the distance of the journey exceeds this, even if the journey itself is accomplished in a shorter time.
The scholars have differed on the length of this distance. Many scholars are of the opinion that it is 16 Farsakh, and each Farsakh equals three miles, thus totalling to 48 miles. (See: Faydh al-Bari ala Sahih al-Bukhari, 2/397)
http://islamqa.org/hanafi/daruliftaa/7865
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